The Design And Strengthening Mechanisms Of Composite Twin Structure In Magnesium Alloy AZ31

Grain refinement is one of the main methods to harden metals. Grain refinment byhigh angle grain boundaries is commonly used, while grain subdivision by coherenttwin boundary is also effective to strengthen metals. Compared to grain boundaries,coherent twin boundaries have better thermal stability, which contributes to applicationsin high temperature environment. The pre-strained single twin structure onlystrengthens Mg alloys along specific directions, while dramatically reduce yieldstrength along other directions. In current project, a hot rolled Mg alloy AZ31thickplate with a basal texture is used. A cross compression along the TD and the RD is usedto prepare a composite twin structure that is composed of {10-12} primary twin and{10-12}-{10-12} secondary twin. The composite twin structure is used to harden Mgalloy in three dimensions. The influence of pre-straining conditions on the volumefractions and size of different structure is studied. Mechanical properties andtension-compression yield asymmetry of samples with different composite twinstructures are systematically addressed, too. In-situ EBSD analysis about deformationbehavior of composition twin structure are carried out. At last, the principle to designcomposite twin structure and the hardening mechanisms of composite twin structure arediscussed. Several conclusions are reached as below:(1) Compared to single twin structure, a composite twin structure is more effectiveto refine grains. The content of matrix, primary twin and secondary twin are highlydependent on the pre-strain along TD and RD. Increasing pre-strain along RD enhancevolume fraction of both primary twins and secondary twins, while reduce fraction ofmatrix. Twin size increases slightly with a higher strain along RD. When the strainalong RD exceeds7%, many grains are totally twinned.(2) Compared to the structure that contains only{10-12} primary twin, thecomposite twin structure is much more effective to harden the samples in threedimensions without obvious drop in plasticity. The volume fractions of twins incomposite twin structure are highly dependent on pre-strain condition and greatly affectthe yield strengths. With suitable pre-strain conditions, the composite twin structure canharden the materials in three dimensions.(3) Both detwinning and {10-12} twinning take place in the samples containing acomposite twin structure due to the presence of multiple types of twins with different orientations. The {10-12}-{10-12} secondary twin also detwin under specific loadingconditions. The detwinning of {10-12}-{10-12} secondary twin proceeds by growth ofthe{10-12} primary twin, in which the {10-12}-{10-12} secondary twin appears. Bytailoring the composite twin structure, detwinning can be totally inhibited during theloading in three dimensions.(4) Compared to the structure containing only {10-12} primary twin, thecomposite twin structure greatly reduces the activity of detwinning. The yield strengthof samples are determined by both volume fractions of structure deformed by twinningand structure deformed by detwinning.(5) Suitable annealing treatment of Mg alloy with pre-strained twin can inducedhardening of detwinning deformation. Our results show that segregation of solute atomsat twin boundary can pin the twin boundary migration leading to enhanced activationstress for detwinning.